The prior art sensor devices typically in instruments such as flow meters have the problem that it has proved impossible to obtain a construction resistant to vibration, temperature, temperature shock and corrosive media. Use is made of strain gauges disposed on the outside of the plate. If piezoelectric sensors are disposed inside the instruments, they are mechanically so connected thereto the sensor has only limited resistance to temperature and shock.
The prior art sensor devices typically include a probe or signal pick up means usually fitted in juxtaposition with a signal generator such as a bluff body of a vortex flow meter, either within it or aligned down stream to it and the sensing element is usually located outside the fluid conduit and secured to the probe. The sensor device provides an output proportional to the deflection or strain generated in the probe by the pressure fluctuations caused by the flow of fluid through the conduit.
Such deflective sensors have the disadvantage that they are unable to discriminate between external vibrations in addition to frequency and amplitude of the signals generated for instance, by the flow of fluid. External vibrations degrade the output signal from the sensor, and restrict the lower end of the useable measurement range of the flow meter. External vibrations are caused, for example, by the operation of nearby pumps, valves, or other process machinery.
A strain detecting system is disclosed in the U.S. Pat. No. 3,972,232, in which a vortex generator of a flexible material was provided and a strain detecting element was bonded thereto isolated from outside by means of a diaphragm, with the chamber being filled with oil. A solid construction can not be attained due to flexibility of the sensor and the diaphragm. Moreover, because of oil, the sensor device could not be operated in a high temperature range.
The piezoelectric effect was discovered by Jacques and Pierre Curie in 1880. They found that if certain crystals were subjected to mechanical strain, they become electrically polarized and the degree of polarization was proportional to the strain applied. The Curies also discovered that these same materials deformed when they were exposed to an electric field. This has become known as the Inverse Piezoelectric Effect.
The piezoelectric effect is exhibited by a number of naturally occurring crystals. For instance, Quarts, Tourmaline etc., and these have been used for many years as electromechanical transducers. For a crystal to exhibit the piezo-electric effect, its structure should have no center of symmetry. A stress tensile or compression applied to such a crystal will alter the separation between the positive and negative charge sites in each elementary cell, leading to a net polarization at the crystal surface. The effect is linear, i.e. the polarization varies directly with the applied stress and direction-dependent, so that the compressive and the tensile stresses will generate electric fields, and hence voltages of opposite polarity. It is also reciprocal so that if the crystal is exposed to an electric field, it will experience an elastic strain causing its length to increase or decrease, according to the field polarity.
Besides the crystals mentioned above, there are several other types of materials available which exhibit piezoelectricity. For example, as applied to the present invention piezoelectric ceramics can be considered. They are obtained by sintering a finely ground powdered mixture, shaped in to required form by compressing—sintering—grinding—electroding—poling. In order to show the piezoelectric properties, the ceramic must be polarized. It is achieved by applying a strong electric field at a high temperature. After polarization, the polar axes of the various single crystals are oriented within a certain solid angle. The direction of polarization between neighbouring weiss domains within a crystallite can differ by 90 degrees or 180 degrees. The piezoelectric effect is used to design sensor elements for sensing pressure, flow, stress and strain and various other parameters. A piezo electrically generated voltage of the same polarity as the poling field occurs, due to compressive forces applied parallel to the poling axis or tensile forces applied perpendicular to the polar axis. Reversing the direction of the applied forces reverses polarity of generated voltages. This property of piezoelectric elements is particularly attractive since no external power is required to be applied to obtain output signals in response to a measurand and the piezoelectric response is quick and very sensitive, making piezoelectric elements excellent transduction elements for sensor devices.
In U.S. Pat. No. 4,248,098 a piezoelectric element composed of lithium niobate (LiNbO.sub.3) is employed for the sensor device. However, the sensor device does not at all tackle ambient noise signals. The arrangement of a single piezoelectric sensor provided on the vortex generator (or receiver), are subjected to a signal transformation. However, disadvantageously, this type of vortex flow meter is adversely affected or influenced, for example, by disturbance vibration, such as piping vibration caused by operation of a pump.
To overcome the in ability of the piezoelectric type sensing element to over come noise signals another arrangement was proposed in U.S. Pat. No. 4,437,350 a vortex flow metering apparatus was proposed comprising a sensor unit having two piezoelectric sensors selectively arranged in the concavity of a vortex generator The two piezoelectric sensors are required to be selectively arranged at two points whereat stress distribution of the noise component due to disturbance vibration and stress distribution of a signal component due to vortex dynamic lift are different from each other. Not only is this design of a sensor device very complicated it requires precise deduction of the spots at which the sensors are to be placed. Since noise signals may emanate from unexpected sources such an arrangement becomes not only expensive but impractical in actually eliminating noise signals. Similar a plurality of piezoelectric elements have been suggested in U.S. Pat. No. 4,835,436 in which is provide a sensor comprising a noise cancelling means, which cancels noise and extracts refined signals by combining two signals respectively generated by a first Piezo electric element with high signal to noise ratio and a second Piezo electric element with low signal to noise ratio. Again, U.S. Pat. No. 4,864,868 suggests two piezoelectric elements as strain transducers which are mechanically clamped adjacent to or into contact with the body where strain is required to be sensed.
Similarly U.S. Pat. No. 6,352,000 and other documents also discuss the use of two or more piezoelectric, capacitive or other elements in an attempt to noise cancellation.
Some other Existing flow meters have attempted to use complex signal processing electronics to extract the frequency signal from the overall sensor output signal. This approach is expensive, complex, and again has only had limited success.
Other flow meters and other measuring instruments such as stress and strain gauges have utilized a separate sensor to detect vibration. The output from this sensor is then subtracted from the overall signal to yield a better desired frequency signal. While this approach has been more successful, the configuration of these instruments have certain inherent disadvantages including that of being very expensive and delicate.
In the prior art probes for Sensors have a flexurally stiff sensor vanes. However, the vanes transmit noise signal along with the required signal and hence this is the main cause of the signal to noise ratio being poor.
In addition, the openings provided in the wall of the measuring tube allow cross flow of fluid effectively weakening of signal takes place. Also, the K factor cannot remain constant over the entire range of flow rates. The wall of the opening becomes weak since the sensor has to be close to the internal wall of measuring tube (pipe).
In tackling noise signals the hitherto approach has been that receiving these noise signals in the sensor element or elements is inevitable and the induced noise signal is filtered out by filtering means or complicated digital signal processing means is used to separate the noise signal from the desired signal. Following this approach inevitably, lower frequency signals go unattended reducing the sensitivity of the instrument and that of the measuring technique as a whole.